The picture portrays the aftermath
of a direct mechanical collision between two galaxies,
as distinct from more distant gravitational interactions.
In the latter, more common situation we customarily
see tidal distortions in one or both galaxies, possibly
with the addition of large scale structures created
by pressure-induced star formation triggered by
the passage of gravitational shock waves through
the gaseous and particulate interstellar media in
one or both galaxies. However, in the case of NGC
6745 pictured here, two galaxies actually collided.

Interacting Galaxy System NGC
6745

When galaxies collide, the stars that
normally comprise the major portion of the luminous
masses of the two galaxies will almost never collide
with each other but will pass rather freely between
each other with little prominent and immediate evidence
in the aftermath that anything has happened. The
reason is that the physical size of individual stars
is so tiny compared with their typical separations
that the probability of physical encounter is vanishingly
small.

However, the situation is quite different
for the interstellar media in the two galaxies -
media consisting largely of clouds of atomic and
molecular gases and of tiny particles of matter,
dust, strongly coupled to the gas. Wherever the
interstellar clouds of the two galaxies collide,
they do not freely interpenetrate but, rather, suffer
inelastic collision. If the relative velocity in
such collisions is sufficiently high, the ram pressure
at the collision interface will produce material
densities sufficiently extreme as to trigger star
formation through gravitational collapse, in much
the same way that star formation is triggered by
the high material densities in gravitationally induced
shock waves.

Because stars in the first few million
years of their life are so extraordinarily luminous
and blue, their distribution is often the most immediate
and obvious indication that there has been a gravitational
interaction - and also, in rare instances, an actual
physical collision. The distribution of luminous
blue stars and clusters of such stars in NGC 6745
indicates such a case.

Using established terminology in describing
the sequence of events, the dominant ('target')
galaxy, galaxy (a), in NGC 6745 is above center
in the illustration, with its nucleus and two attendant
spiral arms about half way between the center and
the top of the illustration. The intruding galaxy,
galaxy (c), came from a region to the left of the
upper-left corner on a gravitationally curved trajectory,
passed on the near side of the nucleus of galaxy
(a) - inducing shock waves in its passage, including
those responsible for the present spiral structure,
collided with the interstellar medium of galaxy
(a) shortly after passing the nucleus, and has now
reached its present location at the lower-right
corner of the illustration.

In this scenario the arc of bright
blue clusters of stars along the upper-right boundary
of galaxy (a) corresponds to the locus of star formation
induced by the propagating gravitational shock from
the passage of galaxy (c). The dense association
of bright blue star clusters seen to be coextensive
with and protruding down and to the right of the
lower terminus of that arc toward galaxy (a) comprises
the stars produced by ram pressure in the physical
encounter between the interstellar media of the
two galaxies. One of the indications that there
actually was such a collision between galaxies (a)
and (c) is that, at high contrast, the blue 'tendrils'
of the star cluster distribution are seen to extend
all the way into galaxy (c). Galaxy (c) seems to
show little evidence of current star formation and
may have been largely shorn of its interstellar
medium.

It will also be noted that there is
an abundance of ruddy structures indicating dust
clouds near the junction between the lower terminus
of the arc and the downward distribution of bright
star clusters, but that these do not extend very
far into this distribution. It appears then that
these stars clusters are simply outrunning interstellar
clouds that did not receive sufficient momentum
exchange from galaxy (a) to keep up. A result will
be that future astronomers 100 million years or
so from now will classify NGC 6745 as a triple system,
three galaxies in a nearly straight line.